Examples of the secondary battery include a nickel-metal hydride battery, a lead-acid battery, and a lithium-ion secondary battery.
Such secondary batteries are used as power sources in portable information terminals typified by mobile phones. In particular, lithium-ion secondary batteries have been actively developed because the capacity thereof can be increased and the size thereof can be reduced.
As examples of positive electrode active materials of a lithium-ion secondary battery, phosphate compounds each having an olivine structure and containing lithium (Li) and iron (Fe), manganese (Mn), cobalt (Co), or nickel (Ni), such as lithium iron phosphate (LiFePO4) lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate (LiCoPO4), and lithium nickel phosphate (LiNiPO4), which are disclosed in Patent Document 1, are known.
In addition, as described in Non-Patent Document 1, a method for evaluating the valence of metal in a metal oxide, and the like by electron energy loss spectroscopy (EELS) is known.